High voltage transmission and distribution lines are typically strung between a series of spaced-apart support towers or poles. The conductors are connected to insulators mounted on or suspended from crossarms extending at the upper end of transmission or distribution poles, or conductor support points built into transmission towers. Periodically it is necessary to replace or repair the poles or towers, crossarms and insulators to maintain the electrical circuit in good working order. It is preferable if this maintenance and repair work can be performed without de-energizing the conductors in order to avoid an interruption of service to consumers, or to avoid the necessity of purchasing power from an alternative source, or other system disruptions.
Hot line repair work is a potentially hazardous undertaking. Safety regulations require that linemen maintain a minimum work clearance or "limit of approach" from energized conductors. The limit of approach varies depending upon the voltage of the conductors in question.
Conventional procedures used by linemen to temporarily support energized conductors in order to enable repair of damaged or obsolete components involve the use of insulated wire tongs, lift poles and rope blocks in labour-intensive, complex rigging arrangements. Conventional fiberglass insulated tools are limited to use only in good weather. Any accumulation of moisture which may impair their insulating property requires that the job be stopped, and that the conductors be placed in an insulator which is rated for all-weather use.
Several auxiliary crossarms have also been proposed in the past for temporarily supporting conductors, thereby reducing the need for labour-intensive "stick work" by linemen. For example, U.S. Pat. No. 4,973,795, which issued to Sharpe on 27 Nov., 1990, relates to an auxiliary crossarm consisting of an insulated boom fitted with polymer insulators and conductor hooks for releasably engaging energized conductors. The Sharpe boom is suspended from a crane above the transmission lines to be serviced.
Auxiliary crossarms for temporarily lifting and supporting energized conductors from below are also well known. Such crossarms typically have sleeves which are connectible to the boom jibs of derrick or bucket trucks.
Utility companies often find it convenient to string both transmission lines and distribution lines on the same pole or tower. The distribution lines are usually suspended between four to twelve feet below the transmission lines. This makes it very difficult or impossible to safely raise prior art boom-mounted auxiliary crossarms to a position immediately beneath the transmission lines in order to provide temporary support to the lower-mounted distribution conductors. Further, the prior art does not disclose any auxiliary arms capable of simultaneously supporting and manipulating both upper transmission lines and lower distribution lines suspended on the same support poles (which is required in the case of pole changes).
Another limitation of prior art designs is that they do not permit pivotal movement of the auxiliary crossarm relative to the boom of a service vehicle. A limited range of pivotal movement is desirable to easily facilitate conductor capture, to enable insertion of the crossarm between upper and lower-mounted conductors and to allow for relocation of conductors to different final configurations (such as from triangular to flat spacing and vice versa). The capacity to control pivotal movement of the crossarm also helps to compensate for "bad parks" by the service vehicle and ensures that the arm can be made level irrespective of the boom angle.
A further limitation of prior art boom-mounted auxiliary arms is that all of the conductors move together as a unit as the truck boom or overhead crane is adjusted. That is, operators are not able to independently control the motion of each separate conductor in order to maximize lineman working clearances or to relocate the conductors to a different spacing configuration.
Accordingly, the need has arisen for a boom-mountable robotic arm for temporarily supporting energized conductors which enables linemen to maximize working clearances in a safe and time efficient manner.